Canada-based Avmax Aircraft Leasing has written to FastJet saying it was removing the registration of three planes leased by its Tanzanian operation. The trio of aircraft, flying under the Fly540 brand, have allegedly run up unpaid leasing and maintenance bills of almost $2m (£1.3m).

The letter from Avmax vice-president Don Parkin, headed "Notice of De-Registration", notified "Fly540/Fastjet of its continuing default" under the lease terms.

It added: "As such, the lessor has elected to deregister the aircraft from the Tanzanian Civil Aviation Registry." The January letter said the deregistration would "be commenced immediately".

Mr Parkin's letter followed a warning in November that Fly540/FastJet was in default and continuing "to refuse and neglect your financial obligations". Avmax claimed the airline had undertaken to pay $150,000 per month for the rental and maintenance of the planes.

Last week's letter was sent to FastJet's chief financial officer Angus Saunders and Don Smith, the majority owner of Five Forty Aviation, which has a separate £4.4m legal dispute with the London-listed FastJet. Five Forty Aviation alleges FastJet has failed to honour a deal to repay debts that the company guaranteed with Chase Bank in Kenya – a claim FastJet refutes.

In a statement FastJet denied it owed Avmax anything, saying: "There has never been a contractual arrangement between Avmax and FastJet plc and therefore FastJet plc does not owe any money to Avmax. Don Smith, through Fly540 in East Africa entered into contractual relationships with Avmax and under these commercial arrangements owes money to Avmax for unpaid aircraft lease rentals and maintenance reserves."

A spokesman for Mr Smith wholly denied that, saying: "The admission document for the listing of Fastjet on AIM states that it owns 90pc of Fly540 Tanzania through its acquisition of Lonrho Aviation (BVI) Ltd. As such, the action taken by Avmax against Fly 540 Tanzania has no implications for Five Forty Aviation or its chief executive, Don Smith."

FastJet also maintained that Avmax's notice related to a single Canadair Regional Jet parked in Nairobi, though the letter refers to three aircraft.

Asked for comment Mr Parkin said: "I am flying to Kenya. We have studied the documents relating to FastJet's acquisition of Fly540 and feel there is a legal link between Fly540 Tanazania and FastJet. The Tanzania business owes us almost $2m. I plan to pursue it in court."

FastJet was formed last June via a £55m all-share deal that saw Lonhro's African aviation wing reverse into Aim-listed Rubicon Diversified Investments. The company was renamed FastJet – a brand owned by Sir Stelios, who was handed a 5pc stake and took a €50,000 (£43,500) per month consultancy fee. Operating in Kenya, Tanzania, Angola and Ghana, FastJet said it would "give people used to 12-hour bus rides the option to fly".